As a promoter, and promoter (originally French promoteur, instigator, initiator), in genetics, a nucleotide sequence of DNA is called, which allows the regulated expression of a gene. The promoter is an essential part of a gene. It is located at the 5 ' end of the gene and hence in front of the RNA coding region. The most important property of a promoter, the specific interaction with specific DNA-binding proteins that mediate the initiation of transcription of the gene by RNA polymerase, and are referred to as transcription factors.
The promoter is part of the " (gene ) regulatory regions ." Among them are equally distant from the gene nucleotide sequences, which may influence its expression yet. So enhancers have a supportive influence on the expression, while silencers reduce this. In the human genome about 775 different promoters are known to date.
Bacterial promoters have a relatively uniform structure, here prevail rather limited differences in the exact nucleotide sequence. One speaks here also sequence dependent of strong or weak promoters. The strength of a promoter can thereby predict by comparison with a consensus sequence from different promoters.
Eukaryotic promoters, are characterized by strong differences among themselves. Although there are there some common elements, such as the downstream promoter element, a general eukaryotic promoter - specific nucleotide sequence is, however, difficult to characterize. Therefore, it is not easy to capture this by bioinformatics methods, for example, in the gene prediction.
Common promoter elements
Sequence motifs that are common in the promoters of a genome are referred to as common promoter elements, as opposed to specific promoter elements which have an importance for the regulation of expression of a particular gene. The general transcription factors bind specifically to the respective general promoter elements. These are either necessary for the initiation of DNA transcription, or represent certain fundamental gene regulation.
The TATA box of the archaea and eukaryotes or Pribnow box of bacteria, examples of a promoter element that occurs in almost all organisms, in a similar form.
The part of a promoter that contains only the common promoter elements which are absolutely necessary for the transcription is the minimal or core promoters.
Often promoter elements are referred to by their distance from the transcription start point, which in this case the term 1 receives while further "upstream" location areas receive a negative sign, further " downstream" lying areas, however, retain their positive sign and cut out the label ± 0 ( zero) remains.
The most important transcription factors in general, bacteria are the sigma factors. The structure of a promoter is therefore particularly dependent on the sigma factors of which it is detected. The best studied are the conditions in which bacterial model organism Escherichia coli. By far the most promoters in E. coli are detected via the factor sigma 70.
Sigma -70 promoter
Based on consensus sequences can be in genes that are transcribed with the help of the sigma- 70 factor, classify following general promoter elements:
- The AT base pair- rich UP element ( for eng. upstream, upstream) above the -35 region,
- The -35 region of the consensus sequence: 5'- TTGACA -3 ',
- The -10 region, and Pribnow box, having the consensus sequence: 5'- TATAAT -3 '.
Meanwhile, there are also indications that the spacer sequences between the -35 - and -10 region recognized by sigma -70 factors and thus they exercise influence on the promoter activity.
It is also true that strong promoters are rich in AT base pairs in front of the start point of transcription. This facilitates the unwinding necessary for the transcription of the double helix, as A-T base pairs form fewer hydrogen bonds than GC base pairs.
The consensus sequences give only a rough indication of the structure of a promoter. A certain sigma 70 - dependent promoter may differ at several points of these sequences.
While the -35 region and Pribnow box are mainly recognized by the sigma factor, the UP element can interact directly with the α - subunit of bacterial RNA polymerase.
Promoters in archaea
Archaea such as bacteria have only an RNA polymerase which, however, homologous to eukaryotic RNA polymerase II. Therefore, the promoter sequences of Archaea are similar to those of eukaryotic RNA polymerase II promoters. However, the promoter structure itself is comparatively less complex in archaea.
Eukaryotes three RNA polymerases, namely, RNA polymerase I, II and III. RNA polymerase I generated rRNA (ribosomal RNA), RNA polymerase II generates mRNA and snRNA ( small nuclear RNA), and RNA polymerase III generated tRNA, snRNA and 5S rRNA. Each RNA polymerase interacts in turn with each of various transcription factors: it recognizes the specific for each polymerase transcription factor binding sites, the promoter region itself and the URS (upstream regulatory sequences). Furthermore, the number of the general transcription in eukaryotes is much larger than bacteria. At the promoter itself can therefore form their own initiation complexes that can affect transcription positively or negatively. This is the reason for the diversity of the promoter sequences that can be recognized by a specific RNA polymerase in conjunction with the respective transcription factors.